1. Field of the Invention
The present invention relates to a lithium secondary battery having excellent cycle characteristics of a battery and excellent battery characteristics such as electrical capacity, storage characteristics etc., and an electrolyte thereof.
2. Description of the Related Art
Recently, small-sized or portable electronic equipments become popular, and therefore, as a driving electric source, the developments of high energy density batteries such as secondary batteries are required. As a powerful candidate thereof, lithium-containing composite oxides such as LiCoO.sub.2, LiMn.sub.2 O.sub.4, LiNiO.sub.2 are noted as a cathodic material since the high electro motive force can be obtained and, as an anode, carbonaceous materials such as coke, graphite are increasingly noted as an anodic material having a high performance and safety, instead of a lithium secondary battery using a metal lithium anode, since there are no occurrence of a short circuit and also no fall-off of lithium from an anode.
However, when the above-mentioned carbonaceous material is used as an anode, there are problems that the battery capacity is gradually decreased on the carbon anode with the increase in the numbers of charge and discharge cycles due to the decomposition of a non-aqueous solvent. For this reason, it is the present status, that the battery characteristics such as the battery cycle characteristics and electric capacity are not necessarily satisfied.
Especially, in the case of a lithium secondary battery using, as a cathode material, the above lithium-containing composite oxide and using a highly crystallized carbonaceous material such as natural graphite and artificial graphite for the purpose of increasing the battery capacity as an anode, the peeling off of the carbonaceous material is observed and these causes problems, depending upon the degree of the phenomenon thereof, that the battery capacity and the cycle characteristics are decreased. The peeling-off of the carbonaceous material is considered to occur due to the decomposition of the non-aqueous solvent in the electrolyte during the charge. This decomposition of the non-aqueous solvent causing the peeling-off is considered to be caused due to the electrochemical reduction of the non-aqueous solvent in the interface between the carbonaceous material and the electrolyte.
For example, although a cyclic carbonate is suitably used as a non-aqueous solvent, when a cyclic carbonate such as ethylene carbonate (EC) is used, the decomposition of the non-aqueous solvent occurs during the repeated charge and discharge cycle to cause the decrease in the battery characteristics. Among the cyclic carbonate, propylene carbonate (PC) having a low melting point and a high dielectric constant is preferable, as the non-aqueous solvent, due to the high electric conductivity at a low temperature. However, when a highly crystallized graphite is used as the anode material, the decomposition of PC becomes remarkable, and therefore, PC was not able to be used as an electrolyte for a lithium secondary battery.